There is a constant stream of new chair designs for office, home, waiting rooms, and the like, which purport to be especially designed for optimum posture of the chair occupant. Typically, the back portion of these chairs is designed to match the curves of the human trunk.
The human trunk has three distinct areas with different curvatures. The lowest, the pelvic girdle itself, is for all practical purposes a monolithic structure although embryologically it is made up of several fused bones. Firmly set into the pelvis, tilted forward at an angle of approximately 30.degree., is the sacral portion of the spine, which can be visibly differentiated on the skeleton but is actually so firmly entrenched that it is usually considered part of the pelvis.
The next higher spinal area of the trunk consists of five lumbar vertebrae between the pelvis and the ribs. They curve forwardly from the highest sacral vertebra at the 30.degree. angle noted above, and then curve backwards to connect with the thoracic, rib-bearing vertebrae. The lumbar vertebral arch is therefore forwardly convex, a status termed "lordosis." From the rear, the lumbar arch is discernible as the hollow of the back. A certain degree of loadosis is normal, but excessive loadosis, due to faulty posture, is well known to be the most common cause of low back pain and sciatica for reasons to be noted.
The third and highest trunk area consists of the thoracic vertebrae, which are part of the rib cage. They arch in the reverse direction from the lumbar vertebrae, being convex dorsally. Dorsad curvature is known as "kyphosis." Some degree of kyphosis in the thoracic spine is completely normal, but excessive kyphosis, characterized by a drooping chest and rounded shoulders, can produce back pain in various areas of the back, either directly or because it influences lumbar and cervical curvatures.
Above the trunk, the cervical vertebrae of the neck region support the head. These bones, too, are normally lordotic, but may also produce local or referred pain if excessively lordotic.
Spinal flexibility is possible because of the spinal disks, cartilaginous pads between each vertebra. They are quite thick and, in fact, take about 25% of the length of the spine. The vertebrae and disks are chiefly unitized by two strap-like ligaments which extend the heights of the spine front and back, and encase the vertebrae like a flexible support stocking. They are known as the Anterior and Posterior Longitudinal Ligaments. Both long ligaments are endowed with sensitive nerves, and they are the principal cause of back pain when they are overstretched by excessive lordosis or kyphosis.
Another frequent cause of pain is contact or pinching of the nerves emerging from the spinal cord, due to excessive spinal curvatures at any point. This produces referred pain in the parts of the body served by the nerve in question. Sciatica is the most common example.
Medical research and experience in relation to back injuries has proven that the minimal stress condition on the spine, the disks and the long ligaments occurs when the spine is straight. In fact, the universal first treatment for spinal injuries is to place the patient flat on his back in a firm bed and even to attach a harness to the hips which rotates the hip forwardly upwards, reducing the angle of the lumbo-sacral joint, and allowing further flattening of the lumbar arch. See Cailliet Low Back Pain Syndrome, F. A. Davis Co., Philadelphia, 1968.
Healthful seating should be designed to encourage low stress posture in the spine, by reducing the various curvatures, and it is therefore an object of this invention to achieve this objective particularly in the utilization of office chairs and other types of work-oriented seating. The invention is particularly directed to the backs of posture-oriented chairs, and discloses a structure that is designed to encourage and urge correct seating posture.
Surprisingly, seat backs have not been properly designed for this purpose. There are many so-called "posture" chairs, but they are characterized by two common fallacies. The most prevalent one is the theory that the lumbar curve has to be supported by the back of the chair. In actuality, the lumbar curve is self-suporting dorsad, which can be demonstrated readily as follows: three stools may be place so that a person lying on them on his back is supported under the shoulders, under the buttocks, and under the heels. It will be readily observed, both subjectively and objectively, that the spinal column supports the body between the first two stools without any assistance or muscular effort.
Since the lumbar curve is self-supporting dorsad, it follows that the best approach to low stress in the lumbar area is to allow the lumbar spine to relax dorsad in the seated position into its own natural minimal curve. This is far better than to support the lumbar spine, which will not result in minimal stress.
The second common fallacy is that the upper back, which is normally somewhat convex dorsad, should be encouraged to remain convex by a matching curvature of the seat back. This, in fact, discourages the proper chest expansion and spinal straightness that should occur so that the upper spine and head balance in an easy, comfortable and healthy way on the middle and lower spine, and stresses are minimized in the thoracic and cervical spine.
Therefore, it is a further objective of the present invention to provide a seat back which corrects the ill-fitting and unhealthy posture effects of the types of backs of chairs presently known and described above.
In the present design, the lower portion of the seat back may be open or recessed so that no support is provided to the lower lumbar spine. Instead, the lumbar spine is allowed to take its own minimal, low stress, curve. This portion of the back design is, therefore, either open or approximately vertical so that it does not contact the relaxed lower spine. A padded area is provided the lower limit of which ranges from six inches to eight inches above the seat near the junction of the upper lumbar spinal bones and lower thoracic spinal bones where the spinal support actually begins.
Above this point or region, a convex, preferably springy, support in the horizontal plane is provided for the thoracic spine and the shoulder blades. A convex support helps or urges the chest to expand, reduces kyphosis, encourages deeper breathing, and tends to align the upper spine so that the head is balanced evenly over it with minimal muscular stress.
Since the trunk of the occupant of the present chair design will not rest heavily against the back, the spring or padding may be modest, and there are many ways to spring or pad the structure.
The preferred structure utilizes a single size of sinuous springs for simplicity in manufacture. In the middle part of the seat back, the sinuous springs are installed flat or in reverse to present a flat or concave curvature (in the horizontal plane) behind the thoraco-lumbar joint. In the thoracic area, the arced sinuous springs are installed in the standard manner by having a convex forward portion providing full use of the arched potential to provide a luxurious but convex support behind the ribs and shoulder blades.
This invention will be more fully comprehended by those skilled in the seating art from the more detailed description that follows including the description of the drawing, and the claims in which modifications and variations as well as equivalents are contemplated.